We have a Diesel Ford Fiesta (Mk4) - manual transmission, and its clutch pedal is very hard to push after a short drive. I.e. in the morning it's ok, but later it becomes very hard to push. I'm no mechanic, but it sounds like a problem with the hydraulics (right?).

Anyone has any ideas as to why this happens and what can I do to fix it?
The car has had 170000km, and as far as I understand, it has a hydraulic pump to aid with the pushing of the clutch.

3 Answers
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I'm not familiar with the Fiesta, but since even much larger cars (with much strong clutch assemblies) don't have hydraulic pumps, I'd expect that it doesn't have one... Should be a simple hydraulic system, basically just a master cylinder, a line, and a slave cylinder down to the actuator. Possible problem with the transmission or the linkage from the slave to the trans/actuator?

As a clutch wears, the pedal action will grow heavier (assuming self-adjusting hydraulic clutch, which is pretty standard now). It should be consistently heavy though, should not change noticeably throughout the day.

Another common problem is boiling the hydraulic fluid, requiring a flush/fill. However, that creates the opposite problem. The clutch pedal gets very light, sometimes sticking to the floor, and the clutch won't disengage.

There are some moving parts involved in the disengaging of the pressure plate after the slave cylinder that may have worn out.

I'm thinking possibly the throwout bearing may be going. When you first start up the vehicle in a cold state it may be operating normally but then while driving it gets warm and gets more resistive (heat causes expansion, probably not good for a worn bearing).

The replacement of the throwout bearing, pivot ball and clutch fork are typically replaced during a clutch job (especially the throwout bearing) so this would explain a similar issue going away for @trip0d199 when their clutch was replaced. Once you've gone through the labor of dropping the transmission out, its worth the extra few bucks to replace those wear and tear parts even if they haven't gone out yet.

How many miles are on this Fiesta? Also, when was the last time the clutch was replaced (if ever)? Clutches in manual transmissions are wear-out parts, and its possible that it might be replaced. I had a manual Dodge Dakota pickup with about 100k miles on it, and it was experiencing the same issue. The resolution - got the clutch replaced, and it fixed the hard-to-push issue.